This invention relates to a problem which occurs when excessively large direct currents (dc) flow in the grounded neutral line of a power transformer (see FIGS. 1 and 2).
Excessively large dc currents can flow in the grounded neutral line of a power transformer when there are large electro-magnetic disturbances present about the power transformer. The core of the power transformer may saturate and depending on the duration of the excessively large dc current flow and the load applied to the transformer, the power transformer may be damaged or destroyed. Accordingly, one aspect of the invention relates to methods and apparatus for detecting core saturation of power transformers due to large electro-magnetic disturbances.
Large electro-magnetic disturbances can result, for example, from geomagnetic storms, or even from nuclear blasts. By way of example, solar flares or storms from sunspot activity follow an 11 year cycle. They begin to increase in intensity every 11 years and peak three to five years after the cycle commences. These storms can affect communication and power systems across the world. Solar flares throw out a cloud of highly charged coronal particles, known as a solar proton event, or “coronal mass ejection” (CME), which hurtle through space. If the cloud is in the direction of the Earth's atmosphere, these particles will be trapped within the Earth's magnetosphere and can cause direct current (DC) flow into any device connected to the Earth. Additionally, many hundred miles of high voltage lines act like an antenna drawing the electro-magnetic pulse from a solar flare toward thousands of transformers on the world's power grids. The absorbed energy can cause many transformers to burn out resulting in disruption and damage to the electrical distribution system and much economic loss including the cost of replacing the burnt out transformers.
The potential difference between the earth and any power apparatus (e.g., transformers, motors, and generators) electrically connected to the Earth (ground) causes a direct current (DC) to flow which may also be referred to as a geo-magnetically induced current (GIC). Where the neutral line of a power transformer is connected to ground, the magnitude of a GIC flowing in the neutral line can vary from a few amperes to several hundred amperes flowing in the earth connection of the power transformer and the current flow can last from several minutes to over an hour. In the case of power transformers having a neutral conductor returned to ground, the GIC (also referred to as INDC) flowing/carried along the neutral conductor can cause the transformer core to saturate.
The probability for core saturation, with respect to the GIC, depends on the design of the transformer with single phase, five legged cores, and shell form designs being the most susceptible. Typically, these types of transformers are the largest in a power system and can present the biggest risk to system reliability should core saturation occur. While three phase core form designs are less susceptible, they too may saturate but at high GIC levels (e.g., over 100 Amps).
Core saturation due to GIC is highly undesirable as the power transformer will become incapable of delivering the required rated power to the load. Also, localized heating and general overheating will occur due to stray flux that induces eddy currents in conductors and metal components within the transformer tank. Such conditions, if allowed to persist without reducing the loading, can lead to catastrophic failure of the power transformer which in turn can affect an entire electric power distribution system.
It is known to monitor the DC neutral current (“INDC”) of a power transformer and to use the amplitude of the current to decide whether to remove or lighten the load being carried by the power transformer. However, relying solely on sensing the level of the INDC is problematic because the INDC does not accurately predict whether core saturation is actually occurring. It is very difficult to model a transformer to accurately predict the level of INDC which will cause core saturation. If only the level of the INDC, which is equal to the GIC is used, it is possible that a transformer operator may make an incorrect decision to shed load too early or too late. Shedding load too early will put a strain on the system especially if multiple units are indicating the flow of excessive GIC. In reality, only a few, if any, of the transformers may be adversely affected. However, not removing the load soon enough may result in catastrophic equipment damage due to overheating which could cause the dielectric integrity of the transformer's insulation system to be compromised.
Using other methods to determine core saturation have not proven to be reliable. For example, measurement of harmonics in the RMS current on the high voltage, low voltage and tertiary windings without monitoring INDC is not a reliable method to determine core saturation. The reason is that specific harmonics associated with GIC can vary depending on transformer design. Some power transformer designs will vary in their harmonic spectrum when subjected to GIC making it extremely difficult to determine if the harmonics are a result of GIC or from the transformer's load or its source. In addition, examining reactive power flow as a means of determining if the transformer core has saturated is not 100 percent reliable as there will always be reactive power (VAR) flow proportional to the magnitude of GIC even though the transformer core is not saturated.
Accordingly, a problem exists in reliably determining when a power transformer is undergoing core saturation as a result of large electro-magnetic disturbances.
Therefore, this invention is directed to method and apparatus for more reliably detecting core saturation of a power transformer due to GIC.